Efficient instance retrieval of subgoals for subsumptive tabled evaluation of logic programs

Abstract

Tabled evaluation is an implementation technique that solves some problems of traditional Prolog systems in dealing with recursion and redundant computations. Most tabling engines determine if a tabled subgoal will produce or consume answers by using variant checks. A more refined method, named call subsumption, considers that a subgoal A will consume from a subgoal B if A is subsumed by (an instance of) B, thus allowing greater answer reuse. We recently developed an extension, called Retroactive Call Subsumption, that improves upon call subsumption by supporting bidirectional sharing of answers between subsumed/subsuming subgoals. In this paper, we present both an algorithm and an extension to the table space data structures to efficiently implement instance retrieval of subgoals for subsumptive tabled evaluation of logic programs. Experiments results using the YapTab tabling system show that our implementation performs quite well on some complex benchmarks and is robust enough to handle a large number of subgoals without performance degradation.

Top-down and Bottom-up Evaluation Procedurally Integrated

“…an entry in a variant table is used to satisfy a subsequent goal if that goal is a variant of the goal that generated the table entry, i.e., equal up to change of variable. In subsumptive tabling (Johnson et al 1999;Ramakrishnan et al 1999;Johnson 2000;Johnson 2002;Cruz and Rocha 2011), a table entry is used to satisfy a subsequent goal if the subsequent goal is subsumed by the generating goal, i.e., is an instance of the generating goal. Note that in such a case the answers to the subsumed goal will (eventually) be in the table of answers for the previously encountered subsuming goal.…”